1. Field of the Invention
The present invention relates to inkjet printing systems. More particularly the present invention relates to a mechanism for cleaning a printhead.
2. Description of the Related Art
Inkjet printing uses a printhead that has a nozzle plate in which an array of nozzles is present. The nozzles eject small droplets of ink for forming an image on a printable substrate.
Inkjet printing systems are used in a wide array of applications such as home and office printers and photo printing but also in industrial printing, including poster printing, signage, packaging, transactional printing etc.
To improve the clarity and the contrast of a printed image, recent research has focused on improvements of the inks. To provide quicker printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images.
A recognized problem in inkjet printers is that the nozzles through which the ink is ejected to the printable substrate can be blocked by clogging of ink inside the nozzles and on the printhead. This can be caused by evaporation of the solvent of the ink at the nozzle location, thereby leaving clusters of pigment particles that clog the nozzle. This renders certain nozzles inoperable and results in deteriorated print quality by the introduction of banding and streaking.
In some industrial applications the required printing speed is so high that it is impossible to rely on evaporation of a solvent or water for the drying of the inks. In that case a solution is offered by the use of UV-curable inks. These UV-curable inks allow for fast solidification under the exposure of high intensity UV-curing lamps. A problem that can occur with this system is that stray-light from the UV-curing source can reach the nozzle plate and can cause solidification of the UV-curable ink near the nozzles, thereby affecting the direction that droplets are jetted and sometimes clogging them.
Other causes of clogging may be dust from dried ink or media fibers (for example paper fibers), or solid particles within the ink itself.
The use of smaller nozzles, which allows for increasing the resolution and the image quality of the print, exacerbates the problem of clogging.
A number of prior art solutions exists for reducing the problem of clogging. These solutions can be used by themselves or in combination.
A first prior art method uses a capping unit. During non-operational periods the printhead can be sealed off from contaminants by a sealing enclosure. This also prevents the drying of the ink. The capping unit usually consists of a rubber seal placed around the nozzle array.
A second prior art method uses spitting. By periodically firing a large number of drops of ink through each nozzle into a waste ink receptacle, commonly called a spittoon, clogs are cleared from the nozzles. This can be concentrated to nozzles which have been identified as being clogged, but usually all the nozzles are actuated during the spitting operation.
A third prior art method uses vacuum assisted purging. During a special operation, in order to clear partially or fully blocked nozzles, a printing cycle is actuated while on the outside of the nozzles a vacuum is applied. This helps clearing and cleansing of the nozzles. The purging is normally performed when the printhead is in a capping unit, because this unit can provide a good seal around the nozzle array for building up the vacuum.
A fourth prior art method uses the application of cleaning fluids. By applying cleaning fluid ink to the nozzle plate, residue on the nozzle plate or within the nozzles is dissolved and the printhead can be cleaned. An example of such a method is found in the publication EP-1 018 430, by Eric Johnson e.a. and having a priority date of 2000 Jan. 6.
Yet another prior art method uses a wiper. Before and during printing the inkjet printhead is periodically wiped clean using an elastomeric wiper, removing ink residue, paper dust and other impurities.
Different combinations of multiple techniques have been known to clean the inkjet printheads.
For example, in the publication U.S. Pat. No. 6,241,337 by Ravi Sharma having a priority date of 1998 Dec. 23, wiping is performed in combination with vibrations and the application and removal of a cleaning fluid. A disadvantage of this method is that the combination of the wiping action with the vibrations has proven to be abrasive for the nozzle plate. This reduces the life of the printhead.
In the publication U.S. Pat. No. 5,557,306 by Tohru Fukushima and having a priority date of 1993 Dec. 15, ink is released from the nozzle plate, the plate is brushed and wiped afterwards. Due to the wiping action wear and tear of the nozzle plate is considerable.
The system described in the publication U.S. Pat. No. 6,164,754 by Daisaku Ide and having a priority date of 2000 Dec. 26 avoids the use of a flat wiper blade by using an elastic cleaning member that fits exactly within a longitudinal groove of the printhead and in which the nozzle section resides. This gives an unsatisfactory result in that the elastic cleaning member may damage the printhead while it is wiping the nozzles.
The technical features that are designed to clean and to protect a printhead are usually located in a service station within the plotter frame. Maintenance of the printhead takes place by moving the printhead to the maintenance station. An example of such a service station can be found in publication U.S. Pat. No. 6,193,353 by Juan Carles Vives and having a priority date of 1998 Mar. 4 where a combination is described of wiping, capping, spitting and purging functions.
A relevant prior art document with regard to the current application is found in U.S. Pat. No. 6,869,161 by Paul Wouters having a priority date of 2002 Jul. 8. This document teaches a method for cleaning the nozzle plate of an inkjet printhead by providing a cleaning fluid to the nozzle plate, by brushing the nozzle plate with a brush in the presence of the cleaning fluid, and subsequently removing the cleaning fluid with the debris by a vacuum.
The above prior art method solves many of the issues of the other prior art techniques in that is gentle on the nozzle plate and avoids wear and tear of the nozzle plate.
However, a problem with this prior art method is that the vacuum is not capable to remove all the cleaning fluid. As a result the excess cleaning fluid and debris can soil the cleaning station and the printhead.
An improved method is therefore required that has the advantages of the method described in the published patent U.S. Pat. No. 6,869,161, but that avoids that excess cleaning fluid is spilled.